monitoring the monsoon in the himalayas: observations in central

1408 VOLUME 131M O N T H L Y W E A T H E R R E V I E W

q 2003 American Meteorological Society

Monitoring the Monsoon in the Himalayas: Observations in Central Nepal, June 2001

ANA P. BARROS AND TIMOTHY J. LANG

Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts

(Manuscript received 5 August 2002, in final form 5 December 2002)

ABSTRACT

The Monsoon Himalayan Precipitation Experiment (MOHPREX) occurred during June 2001 along the southslopes of the Himalayas in central Nepal. Radiosondes were launched around the clock from two sites, one inthe Marsyandi River basin on the eastern footslopes of the Annapurna range, and one farther to the southwestnear the border with India. The flights supported rainfall and other hydrometeorological observations (includingsurface winds) from the Marsyandi network that has been operated in this region since the spring of 1999. Thethermodynamic profiles obtained from the soundings support the observed nocturnal maximum in rainfall duringthe monsoon, with total column moisture and instability maximized just before rainfall peaks. Coinciding withthe appearance of a monsoon depression over central India, the onset of the monsoon in this region wascharacterized by a weeklong weakening of the upper-level westerlies, and an increase in moisture and convectiveinstability. The vertical structure of convection during the project was intense at times, and frequent thunderand lightning were observed. This is suggestive of monsoon break convection, which is expected to be pre-dominant since the monsoon had not fully matured by the end of the month. Comparisons of the MOHPREXdata with the NCEP–NCAR reanalysis data reveal that upper-level winds are characterized relatively well bythe reanalysis, taking into account the coarse model topography. However, moisture is severely underestimated,leading to significant underestimation of rainfall by the reanalysis. The interaction of the ambient monsoon flowwith the south slopes of the Himalayas, modulated by the diurnal variability of atmospheric state, is suggestedas the primary cause of the nocturnal peak in rainfall.

1. Introduction

Background

Atmospheric structure and hydrometeorological pro-cesses along the south slopes of the Himalayas are notwell known or well documented, mainly because of therugged and remote terrain. Also, the mountain rangelies within several poor, developing countries, such asNepal, that do not have the resources to carry out so-phisticated meteorological studies. For example, before2001 no radiosondes had been launched in Nepal formore than 20 yr (M. L. Shrestha, Nepal Department ofHydrology and Meteorology, 2002, personal commu-nication).

Despite these difficulties, however, it is becomingclear that many interesting and important atmosphericphenomena occur in this region. Recent studies haveshown that large rainfall amounts, on the order of 300–400 cm yr21, can fall along the south-facing slopes ofthe Himalayas (Barros et al. 2000; Shrestha 2000; Langand Barros 2002). Studies also have shown that theclimatology of Himalayan rainfall variability differs

Corresponding author address: Dr. Ana P. Barros, Division of En-gineering and Applied Sciences, Harvard University, Pierce Hall 118,29 Oxford St., Cambridge, MA 02138.E-mail: [email protected]

markedly from the rest of the Indian subcontinent(Shrestha et al. 2000). The hydrological importance ofthis rainfall is substantial, as the rivers spawned in thecentral Himalayas are tributaries to the Ganges River,which is a critical water supply for hundreds of millionsof people. In addition, the landslides and erosion causedby this rainfall suggest important geomorphologic im-pacts (Burbank and Pinter 1999).

There have been several studies on the large-scaleeffects of the Himalayas and Tibetan Plateau on theAsian summer monsoon (e.g., Ye 1981; Luo and Yanai1983, 1984; Chen et al. 1985; He et al. 1987; Li andYanai 1996; Wu and Zhang 1998, among others), aswell as their effects on the diurnal circulation in themonsoon region (e.g., Kuo and Qian 1981; Krishnamurtiand Kishtawal 2000). One thing these studies show isthat convection in the Himalayas and Tibetan Plateauplays an important role in sustaining the monsoon,through the release of latent heat. Unfortunately, thesestudies have not expressly focused on the south-facingslopes of the Himalayas, where rainfall has not beenwell quantified until recent years. The modulating effectof south-slope rainfall on the latent heating of the re-gional atmosphere (Magagi and Barros 2003, hereafterMB) and how that impacts the Asian summer monsoonare still unclear, but likely are very important given thelarge rainfall totals.

JULY 2003 1409B A R R O S A N D L A N G

FIG. 1. Topographic map of the Marsyandi River basin, showing locations of the meteorological stationsin the network (numbers; Table 1) as well as the MOHPREX radiosonde launch locations and the Telbrungmicrometeorological tower (Xs). Also shown are various landmarks in the region, including the city ofPokhara. The large-scale map shows the location of the network relative to southern Asia, as well as thelocations of the grid points used in the NCEP–NCAR reanalysis comparison (letters; Table 3).

The question of rainfall impacts in this region and itssurroundings is especially pertinent given that past stud-ies of rainfall in this region, largely based on remotesensing and valley rain gauges, tend to underestimateseasonal totals, particularly for high-altitude ridges(Barros et al. 2000).

b. The Marsyandi hydrometeorological network

A hydrometeorological network was installed in theMarsyandi River basin in central Nepal during thespring of 1999 (Barros et al. 2000). This network lieson several ridges and valleys along the eastern slopesof the Annapurna Range (Fig. 1; Table 1). Beside theaforementioned rainfall totals, both Barros et al. (2000)

and Lang and Barros (2002) noted significant spatialvariability in precipitation (factor of ;4 differences over;10 km distance), which did not show any specificdependence on elevation, particularly at the seasonalscale. In addition, Barros et al. (2000) noted an inter-esting nocturnal peak in rainfall, just past midnight. Oth-er researchers have noted this regional phenomenon aswell (Ohsawa et al. 2001; Ueno et al. 2001; Yatagai2001; Bollasina et al. 2002). This is unusual for moun-tainous regions, which often show an afternoon peak(see Banta 1990 for a review) associated with diurnallyforced upslope flow. While such a peak is visible in thedata (Barros et al. 2000), it often is secondary, partic-ularly at low elevations. One of the more interestingfacets of the nocturnal rainfall peak is that it exists most


TABLE 1. Summary of Marsyandi meteorological network: JJAS,1 Jun–30Sep. NA, not available. Station type: H (high altitude $2000 m), L (low altitude , 2000 m), D (dry station in lee of An-napurnas).

Station no. and name TypeElevation(m MSL)

Jun 2001rain (mm)

2001 JJASrain (mm)

1) Bortung2) Danfedanda3) Ganpokhara4) Khudi5) Koprung

LHHLH

107239872120

8203133

256325868696593

114415503162*23122797

6) Monastery7) Nargaon8) Paiyu Khola9) Pasqam Ridge

10) Pasqam Village

DDLHL

35624220

99329501702

NANA671974NA

NANA

27864279NA

11) Probi12) Purano Village13) Purkot14) Rambrong15) Sundar

LLLHH

14951787

52844353823

529681338206*694

2612790*

12872023*2854

16) Syange17) Tal18) Tansen19) Telbrung20) Temang

LLLHD

12001358152131682760

516292301784175

2117129214693532

821

* Incomplete record during MOHPREX/2001 monsoon.

TABLE 2. Numbers of radiosonde launches for both sites on eachday of the MOHPREX project.

Local date(Jun 2001) Besisahar Tansen

45678

22266

1225

910111213

56667

453

1415161718

88887

1920212223

55555

242526

531

strongly during the summer monsoon; during othertimes of the year the afternoon peak is predominant.This phenomenon is also observed elsewhere in theHimalayas (Ueno et al. 2001).

Monsoon onset in Nepal can be caused by a Bay ofBengal depression that produces very heavy and sus-tained rainfall over 2–3 days (Lang and Barros 2002a).Rainfall at certain Marsyandi stations can exceed 30–40 cm and account for 10%–20% of seasonal totals,particularly at high altitudes ($2000 m MSL). In ad-dition, very strong spatial gradients in rainfall occur, ofthe same order as those that occur on the seasonal scale.

Egger et al. (2000) reported on a field project thatstudied the diurnal cycle of winds in the Kali Gandakivalley (just west of the Marsyandi). They noted anasymmetry in diurnal winds, with daytime upvalleywinds strongly predominating over weak nocturnaldownvalley flow. Other researchers have noted similarphenomena in the Khumbu Valley near Mount Everest(Ueno et al. 2001; Bollasina et al. 2002), with the day-time asymmetry strongest during the monsoon. An in-teresting question is whether this asymmetry holds inthe narrow valleys and convoluted orography that makeup most of the Himalayan range, such as the Marsyandivalley.

It is in this context of pronounced spatial and temporalvariability of precipitation and winds that the MonsoonHimalayan Precipitation Experiment (MOHPREX) wasconceived. The design, goals, and principal results ofMOHPREX will be reported upon in this paper.

2. Design and implementation of MOHPREX

MOHPREX occurred during the period 3–25 June2001. The principal goal was to characterize and un-derstand the spatial and temporal variability of precip-itation along the south-facing slopes of the Himalayas.The variability of precipitation in this region is de-scribed by using data from the preexisting hydromete-orological network, as well as the Tropical RainfallMeasuring Mission (TRMM; Kummerow et al. 1998)and Meteosat-5 (EUMETSAT 2000) satellites. How-ever, understanding of the mechanisms behind this var-iability requires further information on the basic stateof the atmosphere, which can be used to formulate hy-potheses that can be tested by future numerical simu-lations. Hence, the main focus of MOHPREX was thelaunching of radiosondes.

The design of the project focused on key goals: 1)improved understanding of the diurnal cycle of precip-itation, 2) improved understanding of the evolution ofthe atmosphere during monsoon onset, 3) improved un-derstanding of regional-scale spatial variability of mon-soon weather in and near the Himalayas, and 4) im-proved understanding of the coupling between the mon-soon and the hydrological cycle at high elevations inthe central Himalayas (not addressed in this manuscript).To help accomplish these goals, two radiosonde launchsites were defined: Besisahar and Tansen (Fig. 1).

Besisahar was designated as the main site, and thevast majority of balloon flights took place there (Table2). This site is within the north–south-aligned Mar-syandi River valley at 785 m MSL, with ridges in excessof 2000 m MSL on either side. The surrounding to-


FIG. 2. (a) Time series of daily rainfall during Jun 2001, for selected network stations. (b) Jun 2001 meandiurnal cycle of rainfall for high- and low-altitude gauge stations.

pography forms part of the footslopes of the AnnapurnaRange, to the northwest. The main advantages of Be-sisahar were its proximity to the core of the meteoro-logical network (e.g., a gauge station in the town ofKhudi lies just a few kilometers up the Annapurna trek-king trail), and that it had access to good infrastructure(e.g., paved roads, commercial power, and reasonableaccommodations for project workers). Tansen lies 90km to the southwest (SW) of Besisahar, providing anintermediate location between the mountains and theplains of northern India. The launching site was locatednear the top of a small, relatively isolated mountain at1453 m MSL.

The basic frequency of flights, observed at both sites,was five per day: 0000, 0600, 1200, 1800, and 2100UTC. See Table 2 for launch frequency at both sites.Launches were done on UTC time to better match op-erational radiosonde observations and model analyses.Nepal is 0545 ahead of UTC time, so this correspondsto 6-hourly flights with one additional nocturnal flight(2100 UTC, or 0245 LNT) to provide more resolutionaround the time of the postmidnight peak in rainfall. Inthis case, the 1800 UTC launch precedes this peak, andthe 2100 UTC follows it. Launch times periodicallywere adjusted to match overflights by the TRMM sat-ellite, and at times more than five launches per day (atBesisahar) were made as interesting weather conditions

developed. When monsoon onset occurred (later shownto be after 10 June) launch frequency was increased to3 hourly (0000, 0300, 0600 UTC, etc.) at Besisahar tobetter resolve the incoming monsoon flow. This alsoprovided more detailed resolution of the atmosphericdiurnal cycle.

Nepal provided many challenges in order to completea successful field project. Apart from the typical prob-lems associated with developing countries, such as extralogistical strains, poor accommodations, and illness,there were difficult political barriers. The government-mandated mourning period following the tragic demiseof the king and queen of Nepal just as MOHPREX wasslated to start delayed the project. For the first few days,all launches had to be done late at night, resulting in alow frequency of at most two flights per day (Table 2).Information on sounding data quality control can befound in appendix A.

3. Monsoon onset

Daily rainfall at five network stations is shown in Fig.2a. Monthly and seasonal totals can be found in Table1. Unlike the previous 2 yr, there was no major rainevent associated with the monsoon onset, where 1001mm day21 of rainfall fell at a majority of the stationsover 2 days (Lang and Barros 2002). Indeed, after some


FIG. 3. Time series of Besisahar and Tansen winds at four different atmospheric layers.

significant early rainfall, the weather was relatively dryuntil 13 June. The most intense storms after this date(15, 19, and 24 June) produced rainfall accumulationsup to 100 mm in 1 day, but this was a local phenomenonrestricted to one to two gauges at most. However, rain-fall was persistent after 12 June, with most gauges re-ceiving at least some rainfall each day.

Average daily rainfall during June 2001 was 17.0 mmat high altitudes and 19.3 mm at low altitudes. Ten ofthe 17 functioning network gauges during this monthreceived over 500 mm of rain in June (Table 1). Thewesternmost ridge station (Pasqam Ridge) received 974mm, the most of any gauge. Khudi (closest to Besisahar)received 696 mm, but the Tansen rain gauge receivedonly 301 mm. The observed rain totals during June 2001were overall lower when compared with previous years(1999 and 2000). For example, Khudi received 771 mmin June 1999 and 981 mm in 2000, while Tansen had456 mm in June 1999 and 608 mm in 2000. The primaryreason for this was the lack of a major onset rainfallevent in 2001 (Lang and Barros 2002).

Figure 3 shows the temporal evolution of horizontalwinds at Besisahar and Tansen from four atmosphericlayers, centered on 900, 700, 500, and 200 hPa. No-menclature and sign convention are standard, with pos-

itive U being westerly flow and positive V being south-erly flow. The data show that the wind profile underwentconsiderable layer-dependent evolution during June asthe monsoon established itself. For instance, being dom-inated by the diurnal cycle, near-surface winds (900hPa) at Besisahar exhibited no obvious change through-out the 3 weeks of the project, routinely switching be-tween southeast (SE) flow during the day to northwest(NW) flow at night. Tansen also showed considerablevariability but no long-term trends.

At 700 hPa, winds were extremely light during earlyJune, but showed more fluctuations in intensity duringthe middle of the month. Eventually, ESE flow settledin Besisahar, though with strong variability in intensity[east (E) flow being strongest in the morning]. At Tan-sen, however, NW winds became increasingly weaker.Note that Tansen is more exposed at 700 hPa than Be-sisahar, which is situated very close to several 2000–3000-m ridges on the foothills of the Himalayan range(Fig. 1). These ridges serve as an obstacle to flow thatdoes not have a significant southerly component. Whenmesoscale depressions originating from the Bay of Ben-gal interact with easterly vertical shear forced by themountains, an asymmetric secondary circulation devel-ops with strong near-surface upslope (southerly) flow


FIG. 4. (a) Time series of Besisahar and Tansen precipitable water. (b) Same as in (a) but for CAPE.

and return flow (northerly) at midlevel (see also Fig. 6in Lang and Barros 2002).

The largest wind changes occurred aloft at 500 and200 hPa at Besisahar (nearly perfect agreement withTansen at 200 hPa). At 500 hPa, winds originally witha significant westerly component vanished after 10 June,becoming more easterly. A short-duration return to lightwesterly flow occurred after 15 June, but this soon wasreplaced by well-developed SE flow. The most dramaticreduction in westerly flow occurred at 200 hPa with adecrease in wind velocity from U . 20 m s21 at thebeginning of the project to near zero after 11 June. Lightand variable westerly flow eventually prevailed towardthe end of the month. The north–south component at200 hPa was variable throughout MOHPREX. The over-all impression given by Fig. 3 is that of a reduction inthe strength of the westerly component of flow, if notan outright switch to SE winds as the monsoon estab-lished itself.

The time evolution of precipitable water (PW) at Be-sisahar and Tansen is shown in Fig. 4a. While the diurnalcycle was a major component of PW variability, a sig-nificant increase in the mean value of PW between 10and 15 June, remaining more or less constant after that.A similar pattern was found in the values of convectiveavailable potential energy (CAPE; Fig. 4b), a measureof atmospheric instability (Moncrieff and Miller 1976).Finally, the atmosphere warmed during the project, with

an increase in freezing-level altitude on the order of 500m prior to 15 June (not shown).

Based on all these data, it is apparent that monsoononset occurred during the period 10–15 June, mani-festing itself in all notable fields (Figs. 2–4). While theonset process was a multiday phenomenon, we set theapproximate date for onset as 13 June, since by this timeCAPE and precipitable water had reached their maxima,while upper-level westerlies had become nearly quies-cent. In addition, 13 June was the date after which rain-fall was observed on a daily basis in the Marsyandiregion.

This time period is consistent with the developmentand onshore movement of a depression that originatedfrom the Bay of Bengal (Fig. 5), which placed centralNepal under a regime of moist SE flow, consistent withmonsoon onset conditions originally discussed by Langand Barros (2002). Specifically, onset monsoon depres-sions tend to form over the easterly notch of the mon-soon trough over the Bay of Bengal, and propagate west-ward on the northern flank of the westerly jet over theIndian subcontinent: the more southward the jet, theweaker the interaction with the easterly vertical shearalong the Himalayas, keeping the depressions awayfrom the Himalayan range, and favoring disorganizedconvection over northern India. Accordingly, the 2001onset depression moved mostly west from the bay, anddid not interact as strongly with the mountains as did


FIG. 5. NCAR–NCEP reanalysis 850-hPa winds at four different times during the 2001 monsoon onsetin central Nepal.

the onset depressions from 1999 and 2000 (Lang andBarros 2002). Therefore, although the atmosphericstructure exhibited definite change, rainfall totals duringthe 2001 onset period were not as dramatic as in theprevious 2 yr because convection associated with thedepression did not reach the mountains. That is, localprocesses facilitated by the synoptic SE monsoon flowwere more important for rainfall during the 2001 onset.

4. Diurnal cycle

While the month-long evolution of the atmospherewas significant during the MOHPREX project, the pre-vious figures also demonstrate the importance of thediurnal cycle. In fact, for many fields (e.g., CAPE) thediurnal cycle was the strongest component of variability.Thus, further analysis of this variability is warranted.

The diurnal cycle of rainfall during June 2001 isshown in Fig. 2b. High- ($2000 m MSL) and low-(,2000 m MSL) altitude stations were combined andaveraged together in 30-min bins to create this plot.Clearly demonstrated is the near-midnight peak in rain-fall at both types of stations. Also, there is a lull inrainfall at both stations during the time period 0600–1200 local Nepal time (LNT). Average rain rates in-crease during the afternoon hours, although there is asignificant depression in rainfall near 1800 LNT at highaltitudes. The secondary afternoon peak is not as strongduring June 2001. This peak tends to be more apparent

at the seasonal scale (Barros et al. 2000), where dom-inant trends better overcome day-to-day variability.

The methodology employed to calculate the diurnalcycle of sounding-derived variables is reviewed in ap-pendix B. The diurnal cycle of precipitable water isconsistent with a late-morning/midday minimum inrainfall and a postmidnight maximum (Fig. 6a). Themaximum occurs during the period 2100–0000 LNT andthe minimum during 0900–1200 LNT. Soundings thatoccurred when there was no rainfall (see appendix B)show some differences, with a slight dip of 2 mm at0000 LNT compared to the all-weather soundings, anda secondary peak of ;54 mm at 0300 LNT (not shown).

Figure 6b shows the diurnal cycle of CAPE, whichhas a similar pattern to precipitable water, reaching amaximum over the period 1800–0000 LNT. This is the6-h period immediately preceding the nocturnal rainfallpeak in the Marsyandi network. The minimum in CAPEoccurs during the period 0600–0900 LNT, when ob-served cloud cover and rainfall were near their mini-mum. Considering only soundings with no observedrainfall (not shown), there is no significant deviationfrom this general pattern. Overall, the observations areconsistent with a gradual buildup of convective insta-bility during the day, which is then released after mid-night leading to a stable atmosphere at the start of anew daytime period. Total column moisture and insta-bility reinforce one another, and from a thermodynamicperspective the nocturnal rainfall peak is no surprise.


FIG. 6. (a) Diurnal cycle of Besisahar precipitable water. (b) Same as in (a) but for CAPE.

In order to understand this thermodynamic behavior,the diurnal evolution of the vertical profiles of temper-ature and mixing ratio were explored (not shown). At0000 LNT, absolute moisture is well above averagethroughout the depth of the troposphere, while temper-atures are slightly cooler than the daily mean, particu-larly at high altitudes. This is an excellent combinationto produce high instability and precipitable water. Bycontrast, at 1200 LNT, moisture is well below averagein the troposphere and temperatures are average to aboveaverage. Intermediate times show some variability, butoverall reflect the trends between high (0000 LNT) andlow atmospheric instability (1200 LNT). What the datasuggest is normal diurnal heating and cooling of theatmosphere, which is perhaps lagged a bit by the highmoisture levels. Also, there is a daily replenishment ofmoisture occurring at all levels after the nocturnal de-pletion. The timing of these two processes leads to theobserved diurnal evolution of CAPE and precipitablewater.

Note from Fig. 6a that the net daily depletion of at-mospheric moisture is ;6 mm on average. This mois-ture supply must be restored daily through local evapo-transpiration and through advection by the monsoonflow. Figure 7 shows time series of relative humidity,evapotranspiration, and rainfall at Telbrung (station 19,Fig. 1) during June 2001. The hydrometeorological tow-er at Telbrung is equipped with sensors that fully mon-itor the water and energy cycles including soil moisture,

soil temperature, soil heat fluxes, and incoming and out-going shortwave and longwave radiation fluxes. Evapo-transpiration (ET) estimates are obtained indirectly fromthe latent heat flux, which is estimated as the residualof the energy balance equation. During the monsoon,ET varies between 0.5 and 6 mm day21, correspondingto an average daily evaporative fraction (ET/rainfall) onthe order of 0.15. Telbrung is a ridge location; at lowelevations, which do not see rainfall during the day, weestimate that the evaporative fraction correspond to av-erage premonsoon values about 0.35 when soil moistureavailability is not a limiting factor. Although spatiallyvariable, these data imply that 15%–35% of the dailysoil moisture supply is locally or regionally recycled.The remainder must be restored daily through moistureadvection by the monsoon flow. Thus, analysis of thediurnal cycle of tropospheric winds is warranted.

Let us consider the diurnal evolution of U and V inJune 2001 for three surface stations (Fig. 8). The Be-sisahar station was a temporary station established nearthe sounding station to support the flights, whereas Ko-prung and Rambrong are high-altitude stations along theeastern and central ridges, respectively, of the Mar-syandi network (Table 1, Fig. 1). There is a diurnalreversal of wind direction at Besisahar (SE during day-time, NW at night) indicative of normal valley flow,although the strongest winds are observed during thedaytime. By contrast, Rambrong sees no such reversal,only a slight veering of the enhanced daytime winds


FIG. 7. Time series of (top) relative humidity (RH), (middle) evapotranspiration (ET), and (bottom)rainfall observations at the Telbrung tower (station 19 in Table 1 and Fig. 1) during Jun 2001.

FIG. 8. Daily cycles of surface winds at three network stations.


FIG. 9. Daily cycles of vertical profiles of winds at Besisahar.

from SE to SW in the late afternoon. Koprung, at anintermediate altitude, has a very weak reversal in theN–S plane, but once again afternoon winds are strongestand mostly from the S to SE. As noted earlier, the asym-metry between day and night winds is observed else-where in the Himalayas (Egger et al. 2000; Ueno et al.2001; Bollasina et al. 2002).

Figure 9 shows the diurnal evolution of the verticalprofiles of U and V winds at Besisahar. Mean windspeeds are fairly light (,5 m s21) in this region, par-ticularly in the lower troposphere with behavior similarto the surface observations at Besisahar (Fig. 8), thatis, daytime upvalley winds and nocturnal downvalleyflow. The low-level flow is less southerly/more northerlywhen results from rain soundings are considered sep-arately (not shown), suggesting that cool outflow fromrainstorms increases the downvalley tendency of windsin accord with the analysis of Steiner et al. (2003) inthe Alps. The diurnal variability of middle- and upper-tropospheric winds is less clear, mostly showing short-duration fluctuations around the mean trend of increas-ing westerly flow (i.e., veering of winds) with altitudeaway from SE flow in the midtroposphere.

The main reason for the lack of an upper-level diurnalcycle is the evolution of the wind fields during thecourse of the project, which was much stronger than thediurnal cycle amplitude except near the surface (Fig. 3).That is, the most trust should be placed in the diurnalcycle results from the lowest parts of the troposphere,and regions aloft should be viewed skeptically since

temporal evolution of the wind field was not removedwhen computing diurnal cycle.

These results show that a southerly component of flowis present during much of the day (also observed at high-altitude surface stations; Fig. 8), which is strengthenedby the daytime valley wind at low levels. Given thenegative gradient in atmospheric moisture between Indiaand the Tibetan Plateau during the monsoon, near-con-stant advection of moisture into the Marsyandi regionis expected under these wind conditions (Peixoto andOort 1992). Increased near-surface southerly flow dur-ing the afternoon enhances this advection and, in concertwith the weak nighttime wind reversal, sets up the noc-turnal maximum in precipitable water.

5. Case study: 13 June 2001

An excellent example of nocturnal convection is thethunderstorm case of 13 June 2001 (local time). Whilenot the most significant rain event during MOHPREX,a TRMM overpass occurred during this storm, providinginsight into the vertical structure of nocturnal convec-tion in this region. This storm occurred during the mon-soon onset period, when the Bay of Bengal depressionwas slowly progressing westward across central India.Soundings at 1800 (prior to event) and 2100 UTC (dur-ing event) contained 2051 and 2276 J kg21 of CAPE,respectively, which is suggestive of intense convection(Bluestein 1993). Figure 10 shows rain rates spread outduring the night and into the early morning hours for


FIG. 10. Time series of rainfall at select network gauges for the nocturnal precipitation event of 13Jun 2001. The time of the TRMM overpass is noted by the vertical dotted–dashed black line.

available network stations. (Also noted is the time ofthe TRMM overpass.) The Ganpokhara (central ridge)and Pasqam Ridge stations received the heaviest rainfall.However, rain rates remained less than 10 mm h21 atmost gauges.

Although the TRMM overpass occurred during a timeof reduced rainfall at the network, there was still intenseconvection in the region (Fig. 11). In fact, during theentire 2100 UTC Besisahar flight, which matched thetiming of the TRMM overpass, observers noted light tomoderate rainfall. Two vertical cross sections are shownin Fig. 12: one through a core just east of Besisahar(bottom-left panel), and another through the bent lineof convection to the SE (bottom-right panel). The Be-sisahar cell demonstrates the typical vertical structureexpected of electrified tropical convection (Petersen etal. 1996). Peak radar reflectivities exceed 45 dBZ, butthe highest reflectivities are situated mostly below thefreezing-level altitude (;5.5 km MSL). However, re-flectivities in excess of 30 dBZ do extend up to 10 km,well above the 2108C layer altitude at ;7 km. Thethreshold of 30 dBZ above the 2108C layer altitude isgenerally regarded as sufficient for electrification (Pe-tersen et al. 1996). Not surprisingly, frequent lightningand thunder were observed at Besisahar.

The SE convective line is much more intense thanthe Besisahar cell, consistent with its larger size. Indeed,40 dBZ extends several kilometers above the freezing-level altitude. While a time history of reflectivity struc-

ture is not available, the bent nature of the convectiveline is suggestive of retarding effects on storm motionby the orography.

Meteosat-5 infrared imagery (not shown) from thistime reveals a classic pattern of nocturnal convectionin the Himalayas that is often observed during the mon-soon. Strong convection associated with the depression(cf. Fig. 5) is present over central India, while northernIndia is very clear. However, multiple cells are linedagainst the Himalayan Mountains, with clear conditionsover the Tibetan Plateau.

This case study shows that intense and organized con-vective systems, isolated from the major monsoon sys-tems over India, can occur along the footslopes of theHimalayas during the monsoon. Such systems can bevertically developed and produce frequent lightning,reminiscent of monsoon break convective storms thathave been studied in other regions of the world [e.g.,northern Australia; Rutledge et al. (1992), Williams etal. (1992), Cifelli and Rutledge (1998)]. This is con-sistent with a monsoon that is in its nascent stages, aswas the case on 13 June.

6. Regional-scale variability

Currently, there are no other radiosonde stations inNepal, although there are several in northern India, with-in a few hundred kilometers of the project site. In ad-dition, there are two active radiosonde stations in Tibet


FIG. 11. Horizontal and vertical cross sections of TRMM-measured radar reflectivity for the 13 Jun 2001 event.

(see MB for a map of radiosonde stations in this region).However, soundings from these regions are well knownto be among the most error prone in the world (Collins2001). During our analysis, we found many clearly er-roneous winds and temperatures, but by far the largestproblem was with the humidity measurements. In thelower troposphere, the air often was measured to be nearsaturation in the Indian soundings, despite the presenceof temperatures in the range 258–358C. This led to clear-ly unrealistic mixing ratios greater than 25 g kg21. Wesuspect that wetting of the relative humidity sensor, par-ticularly during rainstorms, is a chronic problem for theIndian flights. The other problem with the soundingswere the low frequency of flights (at best two per day;0000 and 1200 UTC) and frequent early terminationbefore reaching the tropopause, both of which hamperthe acquisition of statistically relevant numbers for com-parison with the MOHPREX data.

Therefore, in this study we decided to use the NationalCenters for Environmental Prediction–National Centerfor Atmospheric Research (NCEP–NCAR) reanalysis(Kalnay et al. 1996) for comparison with MOHPREX.Reanalysis data are available at 6-h intervals, 2.58 hor-izontal resolution, and 12 vertical levels in the range1000–100 hPa. The caveat is that over the Himalayas,the reanalysis data of interest (temperature, moisture,winds) are of course primarily derived from the afore-mentioned soundings. However, this is after rigorous

error checking and balancing of any physically unre-alistic values by the reanalysis model. The other ad-vantage of using the reanalysis as a basis for comparisonis that the scientific community uses it extensively, andthus the MOHPREX data provide an excellent inde-pendent check on its ability to capture monsoon con-ditions near the extreme Himalayan barrier.

Because of the coarse horizontal resolution of thereanalysis data (2.58), we do not expect good agreementwith the MOHPREX data below the local terrain en-velope. The reanalysis cannot possibly resolve the steepmesoscale and microscale terrain features in the vicinityof the MOHPREX launch stations, which would havestrong effects on the MOHPREX winds through slopeflows and blocking. For example, ridges in excess of2000–3000 m MSL are located near the valley Besisaharsite. Tansen is more exposed near a hilltop, but ridgeson the order of 1000–2000 m MSL are nearby. However,above 700 hPa (;3000 m MSL; the local terrain en-velope) there is a better basis to compare the NCEP andMOHPREX data, since local terrain would have less ofan effect on the dominant large-scale flow at these al-titudes. Thus, we place most of our focus on middle-and upper-tropospheric data and determine how well thereanalysis matches the large-scale environment in theHimalayas.

The first step is to compare the Besisahar and Tansensoundings themselves, to understand variability on the


FIG. 12. Vertical profiles of mean temperature, mixing ratio, and wind differences between Besisaharand Tansen.

TABLE 3. Specifics of the NCEP–NCAR reanalysis grid points usedin the analyses. Note that the elevation is for model topography only,and does not necessarily match the true elevation at that location onEarth.

Letter Lat (8N) Lon (8E) Elev (MSL, m) Description

ABCDE

30.030.027.527.525.0

82.585.082.585.085.0

43515397

8581966

2115

PlateauPlateauHimalayasHimalayasPlains

mesoscale (physical separation of the sites is ;90 km).For all comparisons in this section soundings were firstinterpolated to a vertical grid with 5-hPa spacing. Figure12 shows the mean profile differences between the twosites for all relevant parameters. The sites had 22 sound-ings in common with one another. On the whole, agree-ment is fairly good, suggesting not much atmosphericvariability is occurring on the mesoscale near the moun-tains. There is a tendency for the Besisahar profile tobe slightly cooler (,18C difference) than Tansen.

Tansen saw greater moisture in the lower troposphere,possibly due to the presence of surface vegetation at theelevated launch site (;850 hPa altitude). However, therewas significantly more moisture (;2 g kg21 difference)in the midtroposphere above Besisahar. One reason forthis may have been the larger percentage of rain flights

at Besisahar, leading to its sondes traveling throughmore cloud layers. However, another possibility couldbe enhanced moisture convergence at midlevels due tothe blocking effects of the Himalayan range. At Besi-sahar there is a tendency for stronger low-level easterlywinds, as well as more southerly flow throughout thetroposphere (except near the surface). The low-levelwind effects are most likely the result of the Besisaharvalley wind.

In comparing the NCEP–NCAR reanalysis to MOH-PREX, we decided on five grid points for this inter-comparison (Fig. 1; Table 3): two at 30.08N in the Ti-betan Plateau (points A and B), to the NW and NE; twoalong the Himalayas at 27.58N (points C and D), to theeast and west of the launch sites; plus one at 25.08N onthe Indian plains SE of the project site (point E). Table3 shows the relevant information for these grid points.

There is good agreement in the wind time series athigh levels in the atmosphere (Fig. 13a). At 500 hPa(Fig. 13b), above the tips of the ridges by Besisahar, Uwinds match well with nearby grid points (points C andD). However, the reanalysis shows more variability thanthe MOHPREX data. Note how the late June increasein V wind at Besisahar is not captured as well by thereanalysis. Perhaps the best agreement between MOH-PREX and the reanalysis is seen at 200 hPa (Fig. 13b),well above the tallest ridges. Here the changes at Be-


FIG. 13. (a) Time series of 500-hPa winds for Besisahar, Tansen, and five surrounding NCEP–NCARreanalysis grid points (Table 3). (b) Same as in (a) but for 200 hPa.


sisahar and Tansen reflected the large-scale variabilityoccurring in the NCEP data. In particular, note how oncethe monsoon was fully established after 15 June, dif-ferences in the U data occurred among the grid points.A reintensification of the westerlies occurs in the Ti-betan Plateau, while in northern India upper-level east-erlies were dominant. This pattern is indicative of thefamiliar upper-level monsoon high over the plateau(Murakami 1987). In between, weak E–W flow becomesestablished in the Himalayan grid points (C and D) andBesisahar.

Mean differences in temperature, moisture, and windprofiles are shown in Fig. 14a (Besisahar-NCEP) andFig. 14b (Tansen-NCEP). We display characteristic ter-rain elevations in these plots to assist in understandingterrain influences. Most ridges in the immediate vicinityof the MOHPREX radiosonde sites do not exceed 2000–3000 m MSL, although 4000–5000 m are common el-evations in the nearby high Himalayas and Tibetan Pla-teau. Note how both Besisahar and Tansen tended tohave more midlevel moisture than the NCEP grid points.In addition, temperature profiles disagreed the most forthe Tibetan grid points as expected since the plateau isan elevated heat source relative to the surrounding at-mosphere (e.g., Ye 1981). As seen in the wind timeseries, the best agreement between MOHPREX and thereanalysis occurred for the Himalayan grid points. How-ever, there still tended to be stronger southerly flow atBesisahar, throughout much of the troposphere. Thiscould have been due to the N–S valley in which the sitewas located. Flow in the N–S plane through this valleywould be less obstructed than flow over the coarse re-analysis topography. There also could have been somechanneling of the flow by the narrow valley, increasingwind speeds. This would increase moisture advectionand may be a partial cause of the observed large dif-ferences in midlevel moisture.

The midlevel moisture differences have profound im-pacts on total column moisture. Figure 15 shows timesseries of precipitable water for the reanalysis points andfor MOHPREX (cf. Fig. 4a). While PW at Tansen agreesrelatively well (within ;25%) with the moistureamounts at the closest NCEP grid point (point C), Be-sisahar has much more moisture, at times even exceed-ing the precipitable water at the upstream NCEP gridpoint over northern India (point E). Most NCEP profilescapture the decreasing moisture in early June, but theincrease associated with monsoon onset comes up to 4days later than during MOHPREX. In addition, there isa late June moisture decrease in most of the NCEP datathat was not observed at Besisahar. Finally, despite hav-ing four time points per day the NCEP data show adiurnal cycle that is extremely weak compared to theone observed at Besisahar. Overall, there appear to beprofound disagreements between the MOHPREX andreanalysis data in terms of atmospheric moisture.

According to the reanalysis, average June 2001 dailyrainfall in the vicinity of the Marsyandi network (using

point D) was 12.3 mm, compared to the 17–19 mmobserved on an average day at the network proper. In-deed, the observed range of rainfall totals at the networkis comparable to reanalysis-predicted rainfall on the out-skirts of the Bay of Bengal. This is also true for pre-cipitable water values. Clearly, significantly more mois-ture exists along the Himalayas than previously esti-mated.

7. Conclusions

The MOHPREX project has been described, and ma-jor results shown. The project was a great success con-sidering the difficulties experienced. Good coverage ofthe diurnal cycle of the atmosphere (temperature, mois-ture, winds), as well as the onset and development ofthe early monsoon, were obtained for two sites alongthe Himalayas in central Nepal.

The onset of the monsoon manifested itself as a grad-ual increase in total column moisture and convectiveinstability, and a weakening of middle- and upper-tro-pospheric westerly winds. The lower-level winds weremore strongly affected by the diurnal cycle of slope andvalley winds. However, daytime upslope/upvalley windswere more intense than their nocturnal downslope/downvalley counterparts. This result is consistent withobservations in other parts of the Himalayas (Egger etal. 2000; Ueno et al. 2001; Bollasina et al. 2002) andsupports the theory of diurnal winds along tropicalslopes in opposing flows (Fitzjarrald 1984). In this case,the opposing flow is the monsoon winds. Also, the ob-servations are consistent with the presence of the Ti-betan surface low pressure (Murakami 1987), whichwould tend to favor upslope flow over downslope (Zanglet al. 2001).

The diurnal cycles of moisture and instability supportthe observations of a postmidnight maximum in rainfallin this region. A case study of a nocturnal storm wasshown, with intense vertical structure as revealed byTRMM. The data suggest that vertically intense breakconvection was prevalent during the early monsoon,consistent with a monsoon flow that had not fully es-tablished itself by the end of the project in late June.

Based on the results of the MOHPREX project, apossible mechanism to explain the nocturnal peak inrainfall is shown in Fig. 16. Large-scale monsoon flowis presumed to be roughly constant during the day andnight, though it is of course subject to the variability inthe monsoon trough over northern India and the ap-pearance of monsoon depressions in the Bay of Bengal.The Besisahar wind data show weak diurnal variationwell above the surface, which supports this assumption.However, the mountains provide a huge barrier to theseflows, tending to block them and cause convergence,particularly at low levels. This convergence is reducedduring the daytime because diurnally forced upslope andupvalley flow reduce the spatial gradients in wind ve-locities. The upslope flow aids the formation of con-


FIG. 14. (a) Vertical profiles of mean temperature, mixing ratio, and wind differences between Besisaharand five surrounding NCEP–NCAR reanalysis grid points. (b) Same as in (a) but for Tansen and the reanalysisgrid points. The horizontal dotted lines note the pressure-coordinate heights of characteristic terrain ele-vations near Besisahar and Tansen (2000–5000 m MSL).


FIG. 15. Time series of precipitable water for Besisahar, Tansen, and five surrounding NCEP–NCAR reanalysis grid points.

FIG. 16. Schematic diagram of the proposed mechanism to explainnocturnal rainfall in the Himalayas.

vection at higher elevations, leading to the observedsecondary peak in rainfall there.

At night, however, wind speeds become relativelyweak near the surface, even reversing direction in val-leys such as the Marsyandi. Thus, near-surface windconvergence increases, leading to upward motion thatcan force convection. This convection is aided by thesteady increase of moisture during the day from advec-tion by the monsoon flow. In addition, atmospheric in-stability is maximized during nocturnal hours, due tointeraction between the steady increase in surface mois-ture and diurnal heating and cooling of the atmosphere.The near-coincident availability of moisture, instability,and surface forcing leads to the nocturnal peak in rain-fall. This peak will be strongest at low elevations whereblocking of flow is most active, as observed.

This is qualitatively similar to the processes that causepredominantly nocturnal rainfall over tropical islands,such as Hawaii (Chen and Nash 1994). On tropical is-lands and in the Himalayas, the thermally forced diurnalflow interacts with the predominant environmental flow(trade winds in the case of Hawaii, the monsoon flowfor the Himalayas), causing nocturnal convergence andsubsequent precipitation.

The nocturnal convergence could be aided by the in-teraction between mountain-forced gravity waves andthe thermodynamics of the Himalayan atmosphere,which MOHPREX shows is most favorable for noctur-


nal convection. In a numerical simulation of monsoononset in this region, Barros and Lang (2003) showedthat spatially fixed gravity waves form over the terrain,with downward motion strongest over ridges and up-ward motion strongest between ridges. Under normalmonsoon conditions, the SE flow that creates this gravitywave pattern is not as strong, but it should be enoughto force some waves. The waves occur as long as theforcing flow holds, and the upward-moving antinodesshould fire off convection when the thermodynamics arefavorable (near midnight).

In summary, the proposed mechanism suggests a ro-bust atmospheric system that strongly favors nocturnalconvection and rainfall. It also explains why this noc-turnal peak does not occur elsewhere, such as the Ti-betan Plateau or the northern Indian plains, and whythis nocturnal peak only occurs during the monsoon.The interaction of the ambient monsoon flow with thesouth slopes of the Himalayas, modulated by the diurnalvariability of atmospheric state, is the primary cause ofthe nocturnal peak. Future numerical modeling work isplanned to provide more support for this hypothesis.

This hypothesis has advantages over one based on alow-level jet (LLJ), which is used to explain nocturnalconvection in the high plains of the United States (e.g.,Bluestein 1993). Although LLJs are expected to occurnear large mountain barriers at latitudes characteristicof the Himalayas (;308), we find no evidence for di-urnally variable barrier jets in either the MOHPREX orNCEP–NCAR reanalysis data. The Himalayas are adata-poor region, but strong LLJs may be preventedfrom forming in the vicinity of the Marsyandi network(and other portions of the middle Himalayas) due to thesteep and variable topography in this region. In addition,the long-term presence of the monsoon trough overnorthern India may play a role in preventing or alteringLLJs. Synoptically forced barrier jets, however, can beimportant for moisture and precipitation when monsoondepressions approach the Himalayas (Fig. 5; also Langand Barros 2002). However, under these conditions thelarge-scale flow tends to overwhelm any terrain-forceddiurnal effects (Lang and Barros 2002).

The MOHPREX data provided an excellent indepen-dent check on the performance of the NCEP–NCARreanalysis in this normally data-poor region. Overall,the reanalysis data captured well upper-level winds thatwould not be as affected by the steep topography thatthe model is too coarse to resolve. However, moisturewas consistently underestimated by the reanalysis, hav-ing profound impacts on simulated rainfall amounts,which undercut observed values by about one-third. Thelow model resolution and lack of reliable observationsin this region are the most likely candidates to explainthis underestimation of moisture.

Overall, it appears that the south slopes of the Him-alayas still are not well resolved by existing datasets,despite the presence of prodigious monsoon rainfall andlatent heating. The results of MOHPREX and from the

Marsyandi network in general should go a long waytoward addressing these concerns. These data are beingused currently to evaluate simulations produced by acloud-resolving atmospheric model under typical mon-soon onset conditions in the central Himalayas. Theobjective is to elucidate the orographic effects on thespatial distribution of winds and rainfall, and to char-acterize orography–convection interactions and theirimpact on the diurnal cycle of rainfall in the Himalayas.

Acknowledgments. NSF provided funding for MOH-PREX under Grant EAR-9909498. Several participantswere also funded by NASA/TRMM under Grant NAG5-7781. Dr. Ana Barros was the principal investigator forMOHPREX. Dr. Timothy Lang was the lead scientistin the field during the project. The authors extend theirgratitude to the Nepal Department of Hydrology andMeteorology for the opportunity to conduct the MOH-PREX project. This project could not have been possiblewithout the dedicated efforts of Tank Ojha, our Nepaleseliaison, and his network of interpreters, porters, anddrivers. Dr. Jaakko Putkonen of the University of Wash-ington participated in the first 2 weeks of the field phaseof MOHPREX, and manages the periodic collection ofMarsyandi network data at other times. Timothy Limof NCAR was the lead technician in the field for thefirst 2 weeks of the project, and was responsible fortraining project participants in the launching of radio-sondes. We thank the graduate and undergraduate stu-dents who assisted in the field: Dr. Steve Nesbitt (Uni-versity of Utah) and Casey Brown (Harvard University),as well as Elizabeth Korb and Martha Fernandes (bothof Tufts University). Dr. Rit Carbonne and Ned Cham-berlain of NCAR, and Dr. Douglas Burbank and BethPratt of University of California, Santa Barbara, assistedin the planning of MOHPREX. Dr. Ramata Magagi ofHarvard University provided a very useful Meteosat-5perspective via satellite phone during the project (sat-ellite data were obtained from the University of Wis-consin/SSEC). Dr. Edward Zipser of University of Utahgave excellent feedback and guidance in the interpre-tation of results. Finally, the staff and ownership of theHotel Sayapatri in Besisahar are commended for theirexceptional hospitality toward project workers.

APPENDIX A

Sounding Data Quality Control

The radiosonde system used was the National Centerfor Atmospheric Research (NCAR) GPS/Loran Atmo-spheric Sounding System (GLASS). The radiosondemodel used was the Vaisala RS 80-15GH. Both of thesesystems are standards within the meteorological re-search community, and more information on them (in-cluding specifications and measurement accuracy) canbe found at the NCAR Atmospheric Technology Divi-sion Web site (http://www.atd.ucar.edu/rtf/facilities/


class/class.html). The GLASS system provides excellentvertical resolution with a time resolution of ;1 s. Windsare measured by the GPS method.

There were 150 sondes originally supplied to theMOHPREX project. However, as is typical of similarfield projects, some attrition in the form of launch fail-ures and data loss occurred. Nevertheless, launch fail-ures were typical to below average as compared to otherfield experiments with the NCAR GLASS system (T.Lim 2001, personal communication). Specifically,launch failures were about 10%–15% of all sondes, mostof these being partial failures with some (often most)data available. There were 121 sounding flights of an-alyzable quality for Besisahar (117 with at least somewind data), and 22 for Tansen (21 with at least somewind data; Table 2). Corrupted data were removed bya combination of standard hydrostatic checks and checkson erratic raw data at the timescale of 1 s. Correctedprofiles were carefully checked against less noisy flightsat similar times in order to confirm that the proper datawere retained. No obvious biases in observed fields werenoted.

APPENDIX B

Diurnal Cycle Analysis

There were not enough soundings to compute a di-urnal cycle at Tansen with acceptable statistical confi-dence in the results. Diurnal cycles for Besisahar sound-ings were calculated in the following way. First, the dayin local time was split up into eight 3-h bins (0000,0300, 0600, 0900, 1200, 1500, 1800 and 2100 LNT).Then, soundings were placed in the appropriate bin ifthey occurred within 1.5 h of the bin time. Once allsoundings were distributed, mean values of all fieldswere calculated for each bin. Because Nepal is nearly6 h ahead of UTC and launches were based on UTCtime, flights did not deviate significantly from the centerof the bin period except when they were adjusted tomatch TRMM overpasses. Note from Table 2 that only11 of 23 project days had more than five launches, andonly 6 days had seven or more launches. The most com-mon off hours were 0900, 1500, and 2100 LNT, soresults from these time periods should be viewed withextra caution, as fewer flights were available for anal-ysis. Diurnal cycle analyses do not take into accountthe month-long evolution of the analyzed fields, whichwas discussed in section 3.

Daily cycles of vertical profiles were calculated byfirst dividing up each vertical profile into 11 segments.Resolution of each segment was 50 hPa between 900and 700 hPa, and 100-hPa resolution above this layerup to 100 hPa. For any specific flight, all data withineach segment were averaged together. Then, each seg-ment was binned in time and the daily cycle calculatedin the manner discussed above.

In order to determine the extent to which the presence

of rain affected results, rain and no-rain flights also weredefined. Rain flights were those that occurred with rain-fall observed at the surface, either at the time of launch,up to 1 h prior to launch, or within 15 min after launch.The main purpose was to identify soundings that oc-curred when the lower troposphere was potentially rainsaturated, which could significantly affect low-levelmoisture measurements (and throw off estimates ofCAPE and PW).

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monitoring the monsoon in the himalayas: observations in central

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